1. Field of the Invention
The present invention relates to a color copying machine, a color printer or a color facsimile apparatus which forms a color image consisting of four color components, yellow, magenta, cyan and black.
2. Discussion of the Related Art
In a conventional digital color image forming apparatus, a black signal is generated based on primary signals of yellow, magenta and cyan corrected in accordance with pigments for a printer or the like and an amount of signals equal to the amount of the black signal is subtracted from the primary signals of yellow, magenta and cyan, employing a method of under color removal (hereinafter, UCR). In this process it is known that parameters such as a degree of gray density of the image to which the black pigment is added, or the amount of the black pigment to be .added greatly affects the quality of a formed color image, an amount of used pigments, or stability in image formation. For example, if a parameter such as the UCR of a high ratio is performed in an image area of low gray density, a total amount of yellow, magenta, cyan and black pigments to be consumed can be reduced, thus reducing the cost and improving the transfer efficiency owing to a decrease in pile height of the pigments. There is also an advantage in the image quality that a gray balance is improved or a ratio of conversion of black letters or the like with only black pigment is increased.
However, there are some cases in which directions as to simple high rate UCR (for example, 100%) causes undesirable edge generation by color components removed by UCR in an image where black letters are located in colored background, for example. FIGS. 8(a)-(c) illustrate problems of black letters in a colored background or a white background. As shown in FIG. 8(a), if a filtering processing circuit for exaggerating an edge is provided after color signal generation, colored edges around black letters in the original image are exaggerated as described above, which causes a problem of producing a rough image. In an original image including a white background and black letters in it, another problem occurs in the case where there are great differences among modulation transfer functions (MTF) of a color scanner for each of red, green and blue signals, or great influences caused by delicate registration errors in scanning. Thus, a colored edge is generated around the black letter as shown in FIG. 8(b), and therefore directions as to simple high rate UCR (for example, 100%) exaggerates the colored edge unnecessarily.
An invention disclosed by Japanese Patent Application Unexamined Publication No. Hei. 1-241978 restrains the occurrence of the colored edge. However, this invention basically relates to a gray shade edge in a white background and cannot be applied to the black letters in a colored background described above.